The level of moisture in air at any time is commonly referred to as relative humidity. Percent relative humidity is the ratio of the actual partial pressure of steam in the air to the saturation pressure of steam at the same temperature. If the actual partial pressure of steam in the air equals the saturation pressure at any given temperature, the relative humidity is 100 percent. If the actual partial pressure is half that of the saturation pressure, the relative humidity is 50 percent, and so forth.
Dew point temperature, also known as condensation temperature or saturation temperature, is a function of the level of moisture or steam that is present in the air, and is the temperature at which air has a relative humidity of 100 percent. Condensation of moisture on a surface occurs when the temperature of that surface is at or below the dew point temperature of air surrounding the surface.
When air having a relatively high content of moisture comes into contact with a surface having a temperature at or below the dew point temperature, steam will begin to condense out of the air and deposit as water droplets onto the surface. At this time, a thin layer of liquid water comprised of small water droplets forms on the surface, creating a visual hindrance or “fog” to an observer looking at or through the surface. Once, formed, the condensation can be dispersed and removed either by raising the temperature of the surface, thereby changing the water into steam, or by lowering the relative humidity of the air surrounding the surface, thereby allowing the droplets to evaporate.
Steam, as a gas, exists in a saturated state at pressures and corresponding temperatures that are predictable and measurable. Notably, the standard for steam's thermodynamic properties, including saturation pressures and temperatures, in the United States and arguably the world, is the ASME (American Society of Mechanical Engineers) Steam Tables. These thermodynamic property tables are readily obtainable from ASME, as well as from engineering texts.
In that steam possesses certain characteristics and traits as a saturated gas that are measurable and exact, equations have been developed that permit the engineer to approximate and predict the properties of steam at a desired set of conditions when its properties are known at a different, or datum, set of conditions. Such an equation, in the case of gas saturation pressures and temperatures, is entitled the Clausius-Clapeyron Equation. This equation, which may be described in several variations, may be best stated for the purposes at hand in the following form:       ln    ⁡          [                        p          2          sat                          p          1          sat                    ]        =                    Δ        ⁢                                   ⁢        H            R        *          (                        1                      T            1                          -                  1                      T            2                              )      where    P1sat is the saturation partial pressure at state 1, in units of psia;    P2sat is the saturation partial pressure at state 2, in units of psia;    ΔH is the heat of vaporization, equal to approximately 755,087.46 (ft-lbf)/lbm for steam;    R is the gas constant, equal to approximately 85.8 (ft-lbf)/(lbm-° R) for steam;    T1 is the temperature at state 1, in units of degrees Rankine; and    T2 is the temperature at state 2, in units of degrees Rankine.
Thus, using the Clausius-Clapeyron Equation, once steam's saturation pressure and temperature are known (the saturation pressure and temperature defining state 1 of the steam), given any other desired temperature, the saturation pressure at this temperature can be calculated to a high degree of accuracy (the temperature and calculated saturation pressure defining state 2 of the steam). Conversely, given any known state 1 conditions, for any desired saturated gas pressure, the saturation temperature can be calculated (the saturation pressure and calculated temperature defining state 2 of the steam).